The Seventh Edition of Unix was released by Bell Laboratories in January 1979, nearly four years after Sixth Edition.
In this interval, the structure of the kernel was significantly altered, with many bugs removed and features added: larger filesystems, less restrictions on user accounts, improved portability. At the user level, new library functions were added (such as the Standard I/O library), and many new applications arrived: awk, cpio, f77, lex, lint, m4, make, sed, tar, uucp. Seventh Edition was really designed for PDP-11 systems with separate Instruction and Data spaces, such as the PDP-11/45. It could be squeezed onto the 11/40 with some loss of performance, but a few applications would not fit into the 11/40's address space. The kernel's image size was around 51 Kbytes, a significant increase over Sixth Edition. Because Seventh Edition required more from the hardware than Sixth Edition, its initial performance was generally poorer. Many people outside of Bell Labs made tuning changes to rectify this. These changes were either passed back to Bell Labs, or were added to systems like 2.8BSD.
For more information about Seventh Edition Unix, see The Unix Time-sharing System: A Retrospective by Dennis Ritchie.